Prior to drilling a wellbore, a study must be made to determine if geological conditions exist that could render the location hazardous for drilling. On land, data for this study may be easily obtained by coring and surface mapping. For a subsea location, it is more difficult and costly to obtain data necessary for such a study. Typically, a combination of a deep towed side scan sonar/sub-bottom profiler survey and a geohazard survey are performed. The deep tow survey data is utilized to map surface and near surface characteristics. The geohazard survey is used to obtain fathometer data to produce a contour map and sparker data to obtain further details about the near sea floor geology. This hazard analysis is required as a sound drilling practice and often as a regulatory prerequisite. Hazards that could affect wild-cat drilling include surface faults, debris slides, landslides, mudflows, salt structures, fluid expulsion features, carbonate build-ups, and shipwrecks and other man made obstructions. For drilling a wellbore for long-term operations, buried landslides could also be significant, but for wild-cat or evaluational drilling, movement of a buried landslide would be sufficiently slow that the purpose of the wellbore would be fulfilled prior to a failure of the well due to the buried landslide.
A near-surface hazard analysis is typically performed by obtaining the two survey data sets, constructing models of surface and near-surface geology, and then studying the morphology for indications that one of the formation hazards may exist, and then studying the near-surface geology to determine if the indications evident from the surface model actually represent a hazard.
Although it has not been common practice, sparker data have recently been replaced with high resolution 3D seismic data. A seismic survey incorporates initiating a shock wave above a subsea surface and recording signals reflected from subsurface discontinuities as a function of time. Continuous interfaces between subsurface rock layers can be found as a function of shock wave travel time by matching signal patterns received at various receiver locations. The nature of reflected signals are indicative of the nature of the various rock layers. By using the information obtained from the reflected signals, velocities for the signals through individual layers are estimated, and the estimated velocities are used to convert time data into the depth estimates.
3D seismic involves collecting data along a plurality of lines at one time. The lines are typically acquired one hundred to three hundred feet apart. With 3D seismic data, a considerably more complete understanding of subsurface features can be achieved.
Sources used to initiate shock waves having a range of frequencies. Relatively low frequency shock waves are needed to obtain data from deep within the formation. High frequency data is useful in determination of geological characteristics but is difficult to maintain below about two to three seconds below the mudline. The mudline is defined as the top of the sea floor, and the terms sea floor and mudline will be used interchangeably herein. "High resolution" seismic data is obtained by enhancing of the high frequency responses. Commercially available computer programs such as CogniSeis FOCUS software and Landmark ITA/Advance software may be used to obtain this high resolution data.
Maps based on high resolution seismic data may be prepared using other commercially available software such as ZYCOR and a plotter such as a Versatec plotter.
It is therefore an object of the present invention to provide a method to identify near-surface hazards wherein 3D seismic data is used to provide a surface rendering that is useful to identify geological features that could represent drilling hazards, and high resolution 3D seismic interpretation is used to further analyze those features to identify existence or absence of near-surface drilling hazards for prospective drilling sites that are not in a geologically complex area. It is a further object to provide such a method wherein deep tow and geohazard surveys are not required.